Sensors employing microelectromechanical systems (MEMS) devices are increasingly used in different applications to detect the movement of an underlying object or the presence of a substance or condition due to their relatively small size and their capability to detect relatively small amounts or changes in the measured item. MEMS devices typically employ a movable, inertial mass or flexible membrane formed with one or more fixed, non-moving structures or fingers. For example, in a MEMS accelerometer, the inertial mass may be suspended in a plane above a substrate and movable with respect to the substrate. The movable structure and the fixed structures form a capacitor having a capacitance that changes when the movable structure moves relative to the fixed structures in response to applied forces or acceleration, such as along a predetermined axis of the device, e.g., x- and y-axes.
For example, commercial MEMS accelerometers that measure out-of-plane or z-axis movement may have electrodes positioned above and/or below the inertial mass to allow measurement of differential capacitance in the z-axis. Designs based on an electrode under the inertial mass typically entail driving, or putting a potential on, the inertial mass and using the electrode under the inertial mass to sense z-axis motion capacitively. However, parasitic capacitance between the electrode and the substrate may undesirably impact the reliability of this measurement.